John, 4-8 weeks is pushing a carnauba, IMO. I usually consider any carnauba, "fairly diminished" after about 3-4 weeks. This is worst case scenario, and I normally use that in So.Cal. Its always been my belief that carnaubas more or less just float on the surface anyway, whereas a Polymer (my favored product) will actually bond to surface pores, thereby increasing durability. Im going on 8 weeks since my last UPP application, and with the 20 or so inches of rain we have had recently, im really surprised by the beading on my P/U. As for looks, I have yet to "see" a difference between say S100 and BF or Platinum/Fourstar. I still have a can of #16 I need to try.
Some poeple will seperate the two by visual preferences, be it warmth, glow, depth etc...I just cant see it.
I guess i seperate it by durability. One thing to consider is normally customers wont see the difference between a carnauba or polymer, but they may notice durability by simple morning dew or light rain beeds.